1. Field of the Invention
This invention relates to an image forming apparatus, such as a printer, a facsimile machine, a photocopier, a multifunctional machine, a plotter, etc., employing an ink-jet printing system, and in particular to an image forming apparatus capable of precisely discharging a small droplet in the ink-jet printing system.
2. Description of the Background Art
As an image forming apparatus, such as a printer, a facsimile machine, a photocopying apparatus, a multifunctional machine, a plotter, etc., an ink-jet printing system is known that includes a liquid discharging head.
A system of driving the liquid discharging head provided in the image forming apparatus is known in which, it is now that a driving signal generator generates a driving signal having not only multiple driving pulses P1 and P3 to P7, for example, to discharge droplets from one or more nozzles, but also a micro-driving pulse P2, for example, only to vibrate a nozzle meniscus without discharging the droplets to maintain the nozzle or nozzles at each printing cycle. As these driving pulses P1 and P3 to P7 include, a resonance-driving pulse is employed to cause liquid to resonate in a pressure chamber and be discharged as a droplets therefrom, as well as a non-resonant-driving pulse is also employed to cause the liquid not to resonate but to be discharged as droplets of large and medium sizes as described in Japanese Patent Application Publication No. 2007-182061 (JP-2007-182061-A).
Also known is a technique in which a driving pulse is generated including a first expansion element P1 that inflates (i.e., expands) a pressure chamber having a fixed capacity, a first discharge element P3 that deflates (i.e., contracts) the pressure chamber inflated by the first expansion element P1 and discharges a droplet therefrom, a second expansion element P5 that expands the chamber again deflated by the first discharge element P3, and a second contraction element P7 that deflates the pressure chamber inflated by the second expansion element P5. Then, a time period “t” starting from a starting end of the first discharge element P1 until the last end of the second contraction element P7 is set to within a range of from ½ to 1 times a natural vibration period Tc of a pressure chamber storing liquid therein as described in Japanese Patent Application Publication No. 2008-037027 (JP-2008-037027-A).
However, when multiple droplets having different sizes, such as a large droplet, a medium droplet, a small droplet, etc., are discharged such that the small droplet is discharged in the next driving cycle (i.e., the next printing cycle) after the large or medium droplets are discharged (in the previous driving cycle), since kinetic energy thereof is weak, the small droplet immediately becomes susceptible to vibration of a meniscus in a nozzle and a velocity and an amount of the small droplet vary when discharged immediately after the large and medium droplets are discharged.